The invention is a laser-diode driver circuit which adapts its parameters so that the output light power of the laser follows accurately the imput video-signal voltage applied to the circuit. Lasers used in printing systems (Xerograpic, film etc.) are modulated during the scanning of a photosensitive medium by the laser beam. The simplest kind of modulation is the binary mode where the laser is turned on and off to produce dots of equal intensity.
More sophisticated printing methods require shades of gray. Amplitude modulating the light output of a laser diode presents some difficulties because the light power versus the drive current characteristics of the laser change due to temperature and aging.
The effect of aging on a laser being used in a gray scale mode can be corrected for, by periodic calibration, while slow variations in ambient temperature can be cancelled by mounting the laser in a temperature-controlled enclosure. Both solutions are expensive and the latter one can not cancel faster thermal effects due to laser self heating which is dependent on the data being printed.
A more effective solution is to drive the laser diode with an adaptive circuit which updates its calibration every scan line.
One method of such calibration of a laser diode is described in: Apparatus for Stabilizing the Optical Output Power of a Semiconductor Laser by Shibagaki et al. U.S. Pat. No. 4,733,398. That method concerns the generation of stabilized light pulses of equal intensity only. The invention described herein updates the calibration circuit at the beginning (or end) of every scan line in a time interval when the laser is turned on for start (or end) of scan detection and then allows the laser to be accurately modulated during the scan line by the input video voltage applied to the laser driver circuit.